Virtual-machine with ovn network, mtu is 1442, ssh login vi/top is stuck

i created a virtual-machine with ovn network, the ovn network mtu is 1442,i login according ssh and execute vi/top command, the ssh connection is stuck. but when is set the mtu to 1200, ssh login the vm, execute vi/top is ok. i confused, why?

Hello Htytion, welcome to Incus :slight_smile:

I bieleve it’s because of OVN, uses encapsulation (GRE or GENEVE if i’m not mistaken), that adds some overhead to ip packets :slight_smile:
Using IPV4+V6 also adds a few bits to the header, and thus, reduces MTU :slight_smile:

Depending on your setup, you might be able to use jumbo-frames on host network, and avoid such issues :slight_smile:

Is it a single node ? Clustered setup ? What’s the network like beetween hosts… plain ethernet, or something a little exotic, like GRETAP/L2TP or something in the like ?

1442 should be the normal MTU for the Geneve tunnel overhead on a 1500 MTU backing network.
But maybe the baking network isn’t really 1500 and so there’s an issue there?

Usually I debug those things by using ping -M do -s SIZE IP where I effectively force ping to generate a packet of exactly the size I want, making sure that anything that exceeds the MTU (1442) fails with a fragmentation needed error and anything that fits within the MTU does go through properly.